The present invention relates to a novel rare earth/iron/boron-based permanent magnet and a method for the preparation thereof. More particularly, the invention relates to a rare earth/iron/boron-based permanent magnet having magnetic anisotropy and suitable for use in the actuator for head driving of computer hard disk drives and a method for the preparation thereof.
Since the debut of neodymium/iron/boron-based permanent magnets proposed by Sagawa et al. and by Croat et al., of which the principal phase is Nd.sub.2 Fe.sub.14 B compound, the rare earth-based permanent magnets of this type have acquired very remarkable improvements in the magnetic properties as a consequence of the extensive development works relative to optimization of the base composition and additive elements as well as to the improvement of the preparation method by which high magnetic properties of the permanent magnets can be derived for the respective magnet alloy compositions.
Various proposals and attempts have been made heretofore in the powder metallurgical process, which is the most widely employed for the preparation of neodymium/iron/boron-based sintered permanent magnets, referred to as the Nd/Fe/B-based magnets hereinafter, directed toward a lower and lower content of neodymium and higher and higher content of iron approaching the stoichiometric composition of the Nd.sub.2 Fe.sub.14 B compound which consists of 11.8% by moles of neodymium, 82.3% by moles of iron and 5.9% by moles of boron, toward a lower and lower degree of oxidation of the magnet alloy, toward high and higher magnetic orientation of the magnet alloy grains by under-taking the compression molding in an increased magnetic field, toward finer and finer metallographic structures and so on. As a result of these improvements applied in combination, a maximum energy product (BH)max of the permanent magnets of this type now has reached a level as high as about 88% of the theoretically possible highest value.
It is, however, the generally accepted impression that improvements of the magnetic properties of the permanent magnets of this type by the modification of the powder metallurgical process or the composition of the magnet alloy for the preparation thereof will sooner or later come at an insurmountable limit. For example, the powder metallurgical process is not suitable for the preparation of a high-performance Nd/Fe/B-based magnet of which the content of iron exceeds the theoretical value 82.3% by moles for the Nd.sub.2 Fe.sub.14 B compound. This is because a high content of iron in the magnet alloy necessarily leads to the formation of the magnetically soft Fe phase which causes reversal of magnetization adversely influencing on the coercive force of the magnet. It is also presumable that a metallographic phase of low melting point of which the content of neodymium is higher than the stoichiometric content with consequent deficiency in the content of iron in the alloy composition forms a molten liquid phase which serves to the occurrence of the coercive force of the type of the nuclei incipience and growth by cleaning the surface of the Nd.sub.2 Fe.sub.14 B grains.
An alternative method to the powder metallurgical method is known for the preparation of a Nd/Fe/B-based permanent magnet having magnetic anisotropy, which is the so-called uniaxial hot-deformation method. In this method, a quenched thin ribbon of Nd/Fe/B-based microcrystals obtained from an amorphous thin ribbon by a heat treatment or quenching at a controlled cooling rate, which is available as a commercial product (MQ1, a product by MQI Co.) is hot-pressed into a magnetically isotropic bulky magnet (MQ2, a product by MQI Co.) which is subjected to a uniaxial hot-deformation treatment by pressing so that the magnetic grains are oriented to align their easy magnetization axes in the direction of pressing to give a magnetically anisotropic Nd/Fe/B-based permanent magnet available as a commercial product (MQ3, a product by MQI Co.).
Needless to say, the degree of magnetic orientation in the above obtained magnetically anisotropic permanent magnet positively depend on the extent of the uniaxial hot-deformation. In this regard, the method of uniaxial hot-deformation thus far developed is successful to accomplish a large maximum energy product (BH)max of the magnet which is as large as about 75% of the theoretically possible largest value.
The above described method of uniaxial hot-deformation, however, has a problem that the composition of the magnet alloy to which the method is applicable is limited because deformation by uniaxial hot-pressing can proceed only in a magnet alloy which permits existence of a liquid phase at the temperature of hot-pressing. Namely, the method is not applicable to a magnet alloy having a chemical composition not to allow formation of a phase of low melting point or of a higher content of iron. These situations have led to a general understanding that the uniaxial hot-deformation method is not suitable when the Nd/Fe/B-based permanent magnet is desired to have magnetic properties superior to those of a magnet prepared by the powder metallurgical method.
On the other hand, so-called nanocomposite permanent magnets are highlighted in recent years in respect of the possibility of accomplishing a great improvement in the magnetic properties of permanent magnets. Namely, a nanocomposite permanent magnet is an integral body having a composite structure made from a magnetically soft phase and a magnetically hard phase integrally intermixed with fineness of 10 nm order and coupled by magnetic exchange coupling. As is evidenced by a simulating calculation and by experiments, a nanocomposite permanent magnet exhibits excellent magnetic properties despite the presence of a magnetically soft phase. Accordingly, it would be within possibility to obtain a nanocomposite permanent magnet consisting of magnetically soft and hard phases and having a high saturation magnetization and a high coercive force, of which the magnetic properties may exceed those of the magnetically hard phase per se, by using base materials having a high saturation magnetization for the magnetically soft phase.
As is known, a rare earth-based nanocomposite permanent magnet can be formed from a combination of a magnetically soft phase including the phases of Fe, FeCo, FeB/FeN-based compounds and the like and a magnetically hard phase including the phases of Nd.sub.2 Fe.sub.14 B, SmCo.sub.5, Sm.sub.2 Co.sub.17, Sm.sub.2 Fe.sub.17 N.sub.x, NdTiFe.sub.11 N.sub.x and other nitrides. It is noted here that the combination of the magnetically soft and hard phases is not limited to one or several of specific combinations but can be any of combinations of the magnetically soft and hard compounds freely selected from the above given species for each of the respective phases. The composition of the magnetically hard phase is not always a limiting factor to the combinations.
While the magnetic exchange coupling between the magnetically soft and hard phase in a nanocomposite magnet can be effective only when the magnetic grains of the phases have a 10 nm order fineness, no successful results have yet been obtained for imparting magnetic anisotropy to a nanocomposite magnet of such an extreme fineness of the structure.
While the feature of nanocomposite permanent magnets consists in a relatively high residual magnetic flux density Br accomplished by the presence of a magnetically soft phase even when the magnet has a magnetically isotropic structure, the coercive force and the maximum energy product of a nanocomposite permanent magnet cannot be high enough when the magnet has a magnetically isotropic structure.
A serious problem in nanocomposite permanent magnets is that a nanocomposite magnet of a bulky form can hardly be prepared. Namely, nanocomposite magnets are prepared usually by the method of melt-quenching or mechanical alloying and obtained in the form of a powder or in the form of a thin ribbon and no practical method has yet been developed for converting such a powder or thin ribbon into a bulky form of the magnet without coarsening of the nanocomposite structure. The only method by utilizing a pulsed ultrahigh pressure for conversion of a powder of a nanocomposite magnet into a bulky form is very specific and expensive and far from the possibility of practical use.
As is discussed above, nanocomposite permanent magnets cannot be under the way of development unless a method is established both for imparting magnetic anisotropy and for preparing a bulky form of the magnet simultaneously.